Does it have to be About.php, or is about also good? The latter case would just require you to enable "pretty permalinks", for which you don't need a plugin. I'm not sure, but I don't think the Redirection plugin will do what you want here: you want the URL to stay the same, and I think the plugin forwards you to another page. Is it OK if request to /About.php are redirected to /about? This way old links still work, and new ones use the built-in WordPress structure.
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Jan FabryDec 7 '10 at 10:15